1. Field of the Invention
The present invention relates to a charged-particle optical system for use in an electron beam instrument (such as a scanning electron microscope) or a charged-particle beam instrument or device (such as an ion beam apparatus typified by an ion microprobe) to focus a beam of charged particles onto a specimen.
2. Description of Related Art
In a charged-particle optical system such as a scanning electron microscope or transmission electron microscope, an aberration corrector is incorporated in the charged-particle optics to enable high-resolution imaging or increase the probe current density. A system for correcting chromatic aberration by means of a combination of electrostatic quadrupole elements and magnetic quadrupole elements and for correcting spherical aberration by means of four stages of octopole elements has been proposed as such an aberration corrector. The principle is introduced in detail in H. Rose, Optik 33, Heft 1, 1–23, (1971); J. Zach, Optik 83, No. 1, 30–40 (1989); J. Zach and M. Haider, Nucl. Instr. and Meth. In. Phys. Res. A 363, 316–325 (1995)).
The principle of the above-described aberration corrector is briefly described by referring to FIG. 6, where the aberration corrector 10, is positioned ahead of an objective lens 7. The corrector 10 has four stages of multipole elements 1, 2, 3, and 4. An aperture baffle 8 is mounted ahead of the corrector 10. The principal plane of the objective lens 7 is indicated by PP.
In this configuration, a beam of charged particles enters from the left side as viewed in the figure along the optical axis LO. The four stages of multipole elements 1–4 are used as electrostatic quadrupole elements. (As used herein, the term “multipole element” refers to a symmetrically arranged set of electrodes or pole pieces. Each “stage” is comprised of a “multiple element.”) These quadrupole elements cooperate with the objective lens 7 to create a reference orbit for the beam of charged particles. The beam of charged particles is brought into focus onto a specimen surface 20. In this FIG. 6, the direction of the optical axis LO along which the beam of charged particles travels is taken as the Z-direction. The orbit Rx of the beam of particles in the X-direction perpendicular to the Z-direction and the orbit Ry in the Y-direction are both schematically drawn on the same plane.
The structures of the multipole elements 1–4 are next described in detail. FIGS. 7(A) and 7(B) show examples of the multipole elements using dodecapole (12-pole) elements. In FIG. 7(A), a multipole element made up of an electrostatic dodecapole element is shown.
In these configurations, voltages can be independently applied to electrodes U1 to U12 forming twelve poles. That is, in these illustrated examples, amplifiers A1 to A12 in a supply portion 105 supply electric potentials to the electrodes U1 to U12, respectively, according to electric potentials supplied from power supplies 101, 102, 103, and 104, respectively. The power supply 101 is used for generating normal and skew dipole fields. The power supply 102 is used for generating normal and skew quadrupole fields. The power supply 103 is used for generating normal and skew hexapole fields. The power supply 104 is used for generating normal octopole fields.
Where the dipole fields or elements are expressed using general words, dipole elements for normal components are used for deflection in the X-direction, while dipole elements for skew components are used for deflection in the Y-direction.
In FIG. 7(B) shows a multipole element made up of a dodecapole element for creating superimposed electric and magnetic fields. In this configuration, the 12 electrodes W1 to W12 acting also as magnetic pole pieces are made of a magnetic material. Coils for supplying exciting currents to the electrodes are mounted on these electrodes W1 to W12. The electric potential and magnetization can be controlled for each of these electrodes (pole pieces) W1 to W12.
The supply portion 105 applies electric potentials to the electrodes W1 to W12. In this illustrated example, amplifiers B1 to B12 in the supply portion 112 supply exciting currents to the respective coils mounted on the electrodes (pole pieces) W1 to W12 according to normal quadrupole power supply 111.
A method of using a dodecapole (12-pole) element to superimposed electric and magnetic fields and using them as dipoles through dodecapole elements of the electric and magnetic field type is shown in M. Haider, W. Bernhardt and H. Rose, Optik 64, No. 1, 9–23 (1982), especially Table 1.
The configuration of FIG. 7(B) is so constructed that the voltages to the electrodes of the multipole elements and the exciting currents to the magnetic pole pieces are controlled independently because the dodecapole element is used as plural multipole elements. Where the used multipole elements are restricted to only quadrupole and octopole elements, for example, the number of power supplies can be reduced accordingly.
FIG. 8 shows various multipole potentials or multipole elements achieved using a dodecapole element. The configurations of FIG. 8 show examples of an arrangement of the electrodes of electrostatic multipole potentials or multipole elements.
Usually, a multipole field generated by the electrostatic potentials or element having functions equivalent to those of a structure having a reference electrode on the X-axis is known as a normal 2n-pole field or element (n=1, 2, . . . ). A multipole field generated by the electrostatic potentials or element having functions equivalent to those of a structure obtained by rotating the normal 2n-pole element through half of the angular pitch (=2π/4n=π/2n [rad] or 90/n [deg]) between the successive electrodes is known as a skew 2n-pole field or element.
Similarly, in the magnetic field type, a multipole field_generated by the magnetic potentials or element having functions equivalent to those of a structure using electrostatic skew 2n-pole electrodes replaced by magnetic pole pieces is known as a normal 2n-pole field or element. A multipole field generated by the magnetic potentials or element having functions equivalent to those of a structure using electrostatic normal 2n-pole electrodes replaced by magnetic pole pieces is known as a skew 2n-pole field or element.
The arrangement of electrodes and magnetic pole pieces of normal multipole elements (or skew multipole elements) is different between electrostatic and magnetic field type, because the directions in which charged particles undergo forces from their fields are selected to be on the same straight line. In the following description, in cases where it is not necessary to discriminate between these electrodes and magnetic pole pieces, they may be simply referred to as multipole elements.
Actual operation using these multipole elements 1–4 is next described by referring to FIG. 6. A normal dipole field or element is a deflector for deflection in the X-direction. A skew dipole field or element is a deflector for deflection in the Y-direction. They are used for axial alignment but their detail description is omitted herein.
Focus adjustment, which can be grasped as a problem occurring when a reference orbit is created, is first described.
A reference orbit can be a paraxial orbit characterized as follows. A quadrupole created by the first stage of multipole element 1 causes the orbit Ry in the Y-direction to pass through the center of a quadrupole created by the second stage of multipole element 2. A quadrupole created by the second stage of multipole element 2 causes the orbit Rx in the X-direction to pass through the center of a quadrupole created by the third stage of multipole element 3. Finally, a quadrupole created by the third stage of multipole element 3 and a quadrupole created by the fourth stage of multipole element 4 cooperate with the objective lens 7 to bring the beam of particles into focus onto the specimen surface 20. Practically, these elements need to be adjusted mutually to achieve complete focusing.
Where the image is not made clear only by focus adjustments in the X- and Y-directions, skew quadrupole potentials may be used in some cases.
Chromatic aberration correction is next described. In this system, in order to correct chromatic aberration, the electric potential φq2 [V] at the electrostatic quadrupole of the second stage of multipole element 2 and the magnetization (or excitation) J2 [AT] (ampere turn or magnetic potential) at the magnetic quadrupole are adjusted so as not to vary the aforementioned reference orbit. With respect to the whole lens system, the chromatic aberration in the X-direction is corrected to zero. Similarly, the electric potential φq3 [V] at the electrostatic quadrupole in the third stage of multipole element 3 and the magnetization (or excitation) J3 [AT] at the magnetic quadrupole are adjusted so as not to vary the reference orbit. With respect to the whole particle optical system, chromatic aberration is corrected to zero in the Y-direction.
Correction of second-order aperture aberrations is next described. In this embodiment, correction of the second-order aperture aberrations using hexapole elements is described. Ideally, second-order aperture aberrations should not be produced. In practice, however, such aberrations occur parasitically to the aberration corrector 10 due to limitations of mechanical accuracies. After correcting chromatic aberration in the X- and Y-directions, second-order aperture aberrations in the X-direction are corrected to zero over the whole lens system by the electric potential ψs2 [V] at an electrostatic hexapole created by the second stage of multipole element 2. Second-order aperture aberrations in the Y-direction are corrected to zero by the electric potential ψs3 [V] at an electrostatic hexapole created by the third stage of multipole element 3. Then, second-order aperture aberrations in the resultant direction of the X- and Y-directions (e.g., a direction making an angle of 30° or 60° to the X-axis) is corrected to zero by the electrostatic hexapoles of the first stage of multipole element 1 and fourth stage of multipole element 4.
Correction of spherical aberration, which can be grasped as a problem occurring in correcting third-order aperture aberrations is next described. Where spherical aberration is corrected, the third-order aperture aberration in the X-direction is corrected to zero over the whole lens system by the electric potential ψ02 [V] at an electrostatic octopole created by the second stage of multipole element 2. Third-order aperture aberration in the Y-direction is corrected to zero by the electric potential ψ03 [V] at an electrostatic octopole created by the third stage of multipole element 3. Then, third-order aperture aberration in a 45°-direction that is the resultant direction of the X- and Y-directions is corrected to zero by the electrostatic octopoles created by the first stage of multipole element 1 and fourth stage of multipole element 4. Practically, alternate iterative adjustments are necessary.
Correction of higher-order aperture aberrations is next described. In this embodiment, correction of fifth-order aperture aberrations is discussed.
Where the contribution of fifth-order aberrations is reduced to a minimum, a method of correcting fifth-order aberrations by a dodecapole element see H. Rose, Optik 34, Heft 3, 285–311 (1971), a method of minimizing the contribution of the fifth-order aberrations by adjusting the sign and amount of third-order aperture aberrations such as Haider, supra, and other methods are available. These methods are not described in detail herein.
The aforementioned theories and experiments have revealed that the prior art, for example, shown in FIGS. 6 and 7 is excellent. However, sufficient considerations have not been given to achieving still smaller probes. The problems with the prior art system are described below.
First, where hexapole components are produced within the aberration corrector due to mechanical inaccuracy, it is necessary to correct second-order aperture aberrations by hexapole fields. In addition, the hexapole components affect fourth-order aperture aberrations, too. Accordingly, where higher resolution should be obtained, fourth-order aperture aberrations contributing much must be corrected before fifth-order aperture aberrations. That is, it is necessary to correct fourth-order aperture aberrations by decapole (10-pole) fields.
Secondly, where the aforementioned hexapole components are produced, it is considered that skew hexapole components are present, as well as normal hexapole components. In the correction of second-order aperture aberrations as already described as the prior art, second-order aperture aberrations in the X-direction are corrected by the normal hexapole fields. In addition, second-order aperture aberrations in the Y-direction are corrected by the skew hexapole fields. If the skew components are present, spherical aberration cannot be completely reduced to zero with only normal octopole fields that correct spherical aberration (third-order aperture aberrations). That is, it is necessary to correct spherical aberration by skew octopole fields.
Thirdly, if the aforementioned second skew hexapole components are present, fourth-order aperture aberrations cannot be completely reduced to zero with only normal decapole (10-pole) fields. That is, it is necessary to correct the fourth-order aperture aberrations by skew decapole fields.
Similarly, if there are skew hexapole components, fifth-order aperture aberrations cannot be completely made zero with only normal dodecapole fields. That is, it is necessary to correct the fifth-order aperture aberrations by skew dodecapole fields.